Question

What is the major difference between half-adders and full-adders?

(a) Full-adders are made up of two half-adders

(b) Full adders can handle double-digit numbers

(c) Full adders have a carry input capability

(d) Half adders can handle only single-digit numbers

The question was asked by my school principal while I was bunking the class.

Enquiry is from Controlled Inverter topic in portion Arithmetic Circuits of Digital Circuits

Answer 1

Right choice is (c) Full adders have a carry input capability

The explanation: Half adders have only two inputs A and B. When we add two 4 bit binary number like 0001 and 0011, then half adder can not be used because if the first bit of both the numbers is 1, then the sum would be 0 and carry would be 1. But this carry can not be added with the second bits addition of the number. So, half adders are useless. But in full adders, one more carry input is present, so that, if carry of one stage is present, it can be added with the next stage as it is done in normal addition. So, therefore, full adders have a carry input capability.